Abstract: Significance determination process (SDP) is a risk inform method to assess the events of nuclear power plants (NPPs) which has been developed by Nuclear Regulatory Commission (NRC) of USA. Through this method the nuclear safety supervisors can screen and assess the events, and get the risk importance degree. In this paper, the developed PSA model was simplified, the second stage determination model of SDP at-power was developed and the model was verified. The verification results show that the model meets the requirements of the nuclear safety supervisors, and can determine the risk importance of events in NPPs quickly and effectively.